American space scientists have once again failed to make contact with the Mars Polar Lander - raising fears that their multi-million dollar mission may have ended in failure.

More than 24 hours after it was scheduled to have reached the planet's surface, scientists have heard nothing - either from the unmanned spacecraft itself or from its two mini-probes, launched earlier.

I feel like I've been stood up on a date

Mars scientist Dave Crisp

But there are still hopes that the silence may be due to technical problems or that the main antenna may be out of line.

The spacecraft had been in good shape and on course just before communications ended on Friday morning as expected before descent.

"At this point, though, we still have a lot of things we can try and we're doing that ... Everybody has the belief that we can still get a signal from the spacecraft."

If mission controllers fail to make contact by Sunday evening GMT they will try to communicate by using the Mars Global Surveyor spacecraft, currently orbiting the planet, as a relay system.

Nevertheless, many privately realise that the longer that the MPL stays silent the less are the chances that it will ever be heard of again.

"So far, I feel like I've been stood up on a date," remarked Mars programme scientist Dave Crisp.

Troubled mission

Scientists sent the MPL to Mars to learn about the planet's climate by studying layers of dust and possibly ice during the 90-day mission.

Mission control waits for a signal anxiously

The probes were designed to separate from the MPL and smash into the surface at about 640km/h (400 mph).

They would then begin operating compact micro-laboratories, collecting and testing subsurface soil samples, heating them in a tiny oven and scanning them to check for signs of water.

The MPL even had a microphone on board to record the first sounds from another world.

Loss of the lander would be a devastating blow to Nasa.

Only 10 weeks ago, the lander's sibling spacecraft, the $125m Mars Climate Orbiter, burned up in the planet's atmosphere because of an embarrassing mix-up over imperial measurements (pounds, feet and inches) and metric units.